1. Field of the Invention
This invention relates to a method for improving the color of containerized green vegetables, particularly to a method of preserving the green color of heat processed canned green vegetables such as green beans and peas.
2. Description of Related Art
Containerized (i.e., canned) vegetables normally must be subjected to heat processing in order to promote sterility and prolong shelf life. While these processes ensure that the vegetables will have an appropriate shelf life and will be safe for human consumption, this processing can result in adverse effects such as changes in the color of the vegetables. In fact, a recent study has suggested that the loss of green color in canned vegetables has led to a reduction in the per capita consumption of these products.
The conventional method for processing containerized green vegetables essentially comprises the steps of blanching the vegetables in water at 60.degree. to 100.degree. C. for 1-5 minutes, followed by packing the vegetables into a container along with an aqueous solution optionally having small amounts of added sugar and/or salt, and thereafter, subjecting the containerized vegetables to a sterilization process. The purpose of the blanching step is to remove occluded or dissolved oxygen from the vegetables (for proper vacuum maintenance after sealing), activate or inactivate certain enzymes that may affect product quality, soften the vegetable tissue to aid in packing, and/or cleanse the vegetables. For green beans, blanching temperatures of 60.degree. to 80.degree. C. are preferred in order to activate the enzyme pectin methyl esterase, thereby providing a firmer product after processing and preventing sloughing. Blanching may also be performed using steam. In this method, the vegetables are placed on a conveyor belt, and transported through a closed tunnel wherein steam is sprayed over the vegetables (usually for 1-5 minutes also). The sterilization step generally involves heating the sealed containers to a high temperature for a predetermined period of time. For example, typical sterilization steps for canned green beans involve heating to a temperature of approximately 250.degree. F. for about 10 minutes (#303 can).
The above-described conventional process for canned green vegetables suffers from several drawbacks, chief among them being a significant loss of the natural green color of the vegetables. The loss in green color is associated with a reduction in the amount of chlorophyll present in the vegetables. In fact, the blanching and sterilization steps sometimes reduce chlorophyll content by as much as 80 to 100%. The principle manner by which the chlorophyll in the vegetables is lost is by the degradation of green-colored chlorophyll to olive-brown pheophytin and pyropheophytin. Obviously, an olive-brown colored vegetable does not appear pleasing to the eye, and consumer acceptance is thereby greatly diminished.
The degradation of chlorophyll within plant tissues is initiated by heat-induced decompartmentalization of cellular acids as well as the synthesis of new acids. The resulting increase in acidity of the vegetables after heating, as demonstrated by decreases in pH, results in the displacement of the central magnesium ion of chlorophyll by hydrogen ions to form olive-brown pheophytin. Further heating can result in the replacement of the C-10 carbomethoxy group of pheophytin with another hydrogen ion resulting in the formation of olive-colored pyropheophytin. Thus, the degradation of chlorophyll into these compounds results in an undesirable olive-brown color for the vegetables.
Various methods have been employed in the past to prevent containerized green vegetables from losing their natural green color due to the degradation of chlorophyll. Since the release and formation of acids within the vegetables directly contribute to the degradation of chlorophyll into pheophytin and pyropheophytin, various attempts have been made to increase the pH of the vegetables. For example, alkalizing agents have been employed to maintain an elevated pH, thereby preventing the degradation of chlorophyll. Some of these methods have even incorporated the alkaline compounds in the interior lining of the cans in order to maintain the proper pH. All of these approaches, however, suffer from numerous drawbacks, including changes in the flavor, odor, and texture of the vegetables. In addition, the alkalizing agents are usually not effective in neutralizing tissue acids over long periods of storage.
Others have attempted to retain the natural green color by processing the vegetables at higher than normal temperatures for shorter periods of time. While this often results in a more pleasing color immediately after processing, pH reductions during storage eventually result in a loss of chlorophyll similar to that seen for vegetables conventionally processed. U.S. Pat. No. 4,104,410 even eliminates the blanching step entirely, replacing this step with a hot water wash at a temperature well below conventional blanching temperatures. In fact, the inventor in this patent stated that the blanching process itself is the principle cause for the loss of color in canned green vegetables.
The other approach commonly used to prevent loss of green color in canned vegetables is to replace the magnesium ion of chlorophyll with another metal. When the magnesium ion in vegetable chlorophyll is replaced by a zinc or copper ion, zinc and copper pheophytins and pyropheophytins are formed. These zinc and copper complexes have a green color similar to that of chlorophyll, and are considerably more stable in acidic solutions. Thus, hydrogen ions will not replace the zinc or copper present in these complexes, and the olive-brown pheophytin and pyropheophytin will not be formed. This "regreening" effect of copper and zinc ions has been known since the 1940's, and several processes have been developed which exploit this effect. For example, U.S. Pat. No. 4,473,591 discloses a process in which zinc or copper ions are provided in the blanch solution. In this process, also known as the Veri-Green process, the vegetables are blanched in the usual fashion however 100 to 200 ppm of zinc or copper ions are present in the blanch water. It is stated that the addition of the metal ions result in vegetables that are greener than those conventionally processed. In addition, it is stated that further enhancement of the color can be achieved by providing an alkaline earth metal compound coated upon the inner surface of the container.
The process disclosed by U.S. Pat. No. 4,473,591, however, also suffers from several drawbacks, chief among them being an inability to precisely control the amount of zinc or copper which will be present in the containerized vegetables. Food additives are highly regulated by the FDA, and in fact the addition of copper ions to vegetables is currently not permitted in the U.S. While the addition of zinc ions to vegetables is permitted by the FDA, zinc is only permitted in green beans up to a level of 75 ppm or less (total concentration of zinc in can). Since the amount of zinc in the final containerized product is difficult to control when zinc ions are added directly to the blanch water, one must usually employ lower than optimal levels of zinc to ensure compliance with the applicable standard.
U.S. Pat. No. 5,114,725 discloses another process by which zinc or copper ions may be incorporated into containerized green vegetables. In this process the well known method of providing zinc or copper ions in the brine solution of canned green vegetables is employed. The vegetables are first blanched under conventional conditions and then filled into cans where a small aliquot of a concentrated zinc solution is added along with the brine solution. The zinc ions are preferably present at a level between 40 ppm and 75 ppm, the upper limit corresponding to the current permissible concentration as set by the FDA. After the cans are sealed, they are held at an elevated temperature of between 125.degree. and 175.degree. F. for 30 to 60 minutes. The canned green vegetables are then subjected to a plurality of thermal treatments. In other words, the sealed containers are subjected to thermal exposure beyond that which is necessary for commercial sterility. This reportedly results in an improvement in the green color of the vegetables.
Unfortunately, all of the prior art processes have drawbacks. For instance, none of the prior art methods utilizing zinc ions have resulted in thermally processed peas with an acceptable, storage-stable green color and a zinc ion concentration below 75 ppm. Adding zinc directly to the blanch solution creates difficulties in controlling the final zinc concentration, and it has also been found that zinc uptake in pea tissue from a brine solution containing zinc ions is small compared to zinc uptake from a blanch solution containing the metal ions.